Method and composition for plasma etching of a self-aligned contact opening

ABSTRACT

A method of forming a self-aligned contact opening in an insulative layer formed over a substrate in a semiconductor device involves etching the insulative layer with at least one fluorocarbon and ammonia.

FIELD OF THE INVENTION

This application is a divisional of application Ser. No. 09/752,685,filed Jan. 3, 2001, which is incorporated herein by reference.

The present invention relates to a process for etching a self-alignedcontact opening in a semiconductor device, and more particularly, to amethod of plasma etching to prevent build up of undesirable polymersduring contact formation. The invention also relates to a compositionuseful in the method of plasma etching described herein, as well as tothe semiconductor structures formed thereby.

BACKGROUND OF THE INVENTION

In the formation of contact openings or vias in semiconductor devicesused to provide metal-to-metal or conductive layer-to-conductive layercontacts, it is often necessary to etch through one or more layers ofinsulative material formed over a substrate. FIG. 1 shows a crosssection of a portion of a semiconductor device 10 in an intermediatestage of fabrication. The integrated circuit wafer section 10 has asubstrate 12. The substrate is formed of a semiconductor material, forexample silicon, or a semiconductor material over an insulator, forexample silicon-on-insulator (SOI). Field oxide regions 13, transistorgate stacks 15, side wall spacers 17 protecting the gate stacks, anddoped regions 19 are formed over the substrate. A layer of insulatingmaterial 21, which is usually a type of glass oxide available in theart, for example, Boro-Phospho-Silicate Glass (BPSG), or silicon oxidematerial such as silicon dioxide or Tetraethylorthosilicate (TEOS) isformed over the substrate 12. The layer of insulating material 21 may,in actuality, be formed as one or more layers of insulating material of,for example, BPSG, TEOS or silicon dioxide. The insulating layer 21 maybe anywhere from a few hundred Angstroms to several thousand Angstromsin thickness. Formed over the insulative layer is a photoresist maskinglayer 23 using available photoresist materials. The photoresist layer 23has a patterned opening 25 corresponding to the outline represented bythe dotted lines shown in FIG. 1. The patterned opening forms theoutline of a self-aligned contact (SAC) opening which is thereaftercreated. The SAC opening will provide access to the substrate 12 throughthe insulative layer 21.

Referring to FIG. 2, a plasma etch is then conducted to form the SACopening 27, using the patterned opening 25 of the photoresist maskinglayer 23 as a guide. The patterned opening 25 generally follows theoutline of the sides of the spacers 17 to align the etch for the contactopening. During the etching process, one or one fluorocarbons areintroduced into a chamber containing the semiconductor device 10. Undersuitable conditions ionic and neutral etchants are then formed to etchthe insulative layer 21 so as to form the opening 27. Unfortunately,under prevailing conditions the reaction of these etchants and otherspecies with the insulative material of layer 21 produces a polymerlayer 29 on the bottom and side wall spacers of opening 27 as a reactionproduct. A thin accumulation of polymer layer 29 along the sides of theside wall spacers 17 may be desirable to prevent subsequent erosion ofthe spacers. However, a build up of polymer layer 29 at the bottom ofthe SAC opening 27 can cause an undesirable phenomenon known as “etchstop”, in which further etching through the insulative layer 21 to thesurface of the substrate 12 is prevented by this polymer layer build up29. In effect, the etch stop polymer layer 29 formed from the insulativelayer can significantly inhibit suitable formation of the contactopening 27.

Attempts have been made to prevent etch stop during contact openingformation. For example, it is known to add oxygen (O₂) to the mixture offluorocarbon gases which are introduced into the reaction chamber. As aresult, the etch rate of insulative material, e.g. oxide, has been shownto increase. The addition of oxygen appears to be accompanied by anincrease in the density of the fluorine atoms in the etchant discharge.However, the use of too much oxygen may undesirably dilute the fluorineconcentration, and thereby decrease the etch rate. Oxygen may also beutilized to clean polymer debris from the bottom of the contact openingafter exposure to the fluorine-based etchant plasma. Nitrogen (N2) hasalso been utilized for cleaning residual debris after the etchingprocess.

What is now needed in the art is a new method of forming a self-alignedcontact opening in a semiconductor structure which can substantiallyeliminate etch stop problems. Also needed is a new composition which canbe utilized in conjunction therewith.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention a method is provided forforming an opening in an insulative layer formed over a substrate in asemiconductor device in which the insulative layer is etched withammonia and at least one fluorocarbon. The process parametershereinafter described will substantially reduce or eliminate theformation of an “etch stop”.

Also in another aspect the invention provides a composition suitable foruse in etching an insulative layer formed over a substrate in asemiconductor device. The composition comprises a gaseous mixture of atleast one fluorocarbon and ammonia.

In another aspect the invention provides a process of forming an openingin an insulative layer formed over a substrate in a semiconductordevice. A patterned photoresist mask layer is first formed over theinsulative layer. A self-aligned contact opening is then etched in theinsulative layer through an opening in the patterned resist layer. Theopening is etched through to the substrate using a combination ofammonia and at least one fluorocarbon.

In another aspect the invention provides a method of preventing etchstop during etching of a semiconductor device which comprises addingammonia to at least one fluorocarbon which is utilized for the etching.

In another aspect the invention provides a method of preserving a sidewall spacer surrounding a gate stack during a self-aligned contact etch.The side wall spacer is contacted with a combination of at least onefluorocarbon and ammonia so as to form a protective layer thereover. Theprotective layer prevents erosion of the spacer as the contact openingis formed through to the substrate upon which the gate stack has beenformed.

In another aspect the invention provides a method of forming aconductive plug inside a contact opening in an insulative layer betweenadjacent gate stacks formed over a substrate in a semiconductor device.The insulative layer is contacted with a plasma etchant mixturecontaining ammonia and at least one fluorocarbon at a pedestaltemperature within the range of about −50 to about 80 degrees Celsius soas to form a self-aligned contact opening in the insulative layerbetween the gate stacks without an etch stop. The contacting furtherforms a protective or passivating (nitrogen containing) layer overopposed side wall spacers which have been formed at the gate stacks. Aconductive plug is then deposited inside the opening such that the plugis separated from the side wall spacers by the protective layer.

These and other advantages and features of the present invention willbecome more readily apparent from the following detailed description anddrawings which illustrate various exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a semiconductor device in anintermediate stage of fabrication.

FIG. 2 is a cross sectional view of the device shown in FIG. 1 in afurther stage of fabrication.

FIG. 3 is a cross sectional view of a semiconductor device whichutilizes the method and composition of the invention.

FIG. 4 is a cross sectional view of a semiconductor device according toa further embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference herein shall be made to the term “substrate,” which is to beunderstood as including silicon, a silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS) structures, doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures. In addition, whenreference is made to a “substrate” in the following description,previous process steps may have been utilized to form arrays, regions orjunctions in or over the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, indium phosphide, or galliumarsenide. The term “substrate” as used herein may also refer to any typeof generic base or foundation structure.

Referring again to the drawings, FIG. 3 illustrates the method of theinvention which mitigates etch stop problems. According to oneembodiment, there is provided a method of forming a contact openingusing at least one fluorocarbon. It is desirable that the contactopening be a self-aligned contact (SAC) opening, that is, an openingwhich is self-aligned between two successive gate stack structures.Preferably, at least two or more fluorocarbons are utilized, and in someembodiments at least three or more may be used as part of the invention.The fluorocarbon(s) may be chosen from those available in the art forplasma etching. Suitable fluorocarbons therefore include at least onemember selected from the group consisting of fluorinated carbons,fluorohydrocarbons, chlorofluorocarbons and chlorofluorohydrocarbons.Non-limiting examples include such compounds as C₄F₈, C₄F₆, C₅F₈, CF₄,C₂F₆, C₃F₈, CHF₃, and CH₂F₂, and the like. Preferably, one or more ofthe compounds CF₄, CHF₃, and CH₂F₂ are utilized.

The fluorocarbon(s) are introduced with ammonia (NH₃) in a reactionchamber 30 together with the portion of the semiconductor device shownin FIG. 3. The chamber may be any suitable reaction vessel available forplasma etching. The ammonia may be obtained from any suitable source. Ithas now been shown that the combination of at least one fluorocarbon,together with ammonia, is not only effective in forming the contactopening 27 shown in FIG. 3, but is also effective in mitigating againstetch stop, i.e. the problem illustrated in FIG. 2.

The fluorocarbon(s) and ammonia are introduced into a suitable reactionchamber along with the semiconductor device. The reaction chamberpedestal may be set at an operating temperature within the range ofabout −50 to about 80 degrees Celsius, with about 0 to about 80 degreesCelsius being preferred. Operating pressure is typically within therange of about 30 to about 60 milliTorrs, with about 40 to about 50milliTorrs being more preferred, and about 45 milliTorrs beingparticularly desirable. About 600 watts of power is typically applied tothe reaction chamber, but the wattage can vary within a range of about500 to about 1500 watts.

The fluorocarbon(s) and ammonia are introduced into the reaction chamberat a flow rate which will both allow formation of the self-alignedcontact (SAC) opening 27 and prevent or reduce etch stop problems. Insome embodiments, elimination or reduction of the etch stop problem maybe quantifiable by the reduction in time it takes to complete formationof the opening, for example. The flow rates may vary slightly, but aratio of the flow rate for each fluorocarbon to the flow rate of ammoniashould typically be within the range of about 2:1 to about 40:1 (withflow rate being measured as scc/minute or sccm). It is preferred thatthe flow rate ratio not be less than about 3:1. More preferably, theflow rate ratio should be within the range of about 3:1 to about 20:1,and even more preferably about 4:1 to about 10:1.

Actual flow rates for each of the individual fluorocarbon(s) utilized toform the SAC opening 27 will usually be within the range of about 10 toabout 50 sccm, with about 10 to about 40 sccm being preferred. The flowrate will vary according to the particular fluorocarbon being utilized,and different fluorocarbons may have different flow rates. For example,when CF₄ is utilized, a flow rate of about 15 to about 20 sccm ispreferred, with about 16 to about 18 sccm being more preferred. WhenCHF₃ is utilized, a flow rate of about 35 to about 45 sccm may bepreferred, with about 37 to 42 sccm being even more preferred. WhenCH₂F₂ forms part of the etchant plasma, a flow rate of about 10 to about15 sccm is preferred, with about 11 to about 14 sccm being morepreferred. In some embodiments of the invention, it may be desirable toutilize at least two of the foregoing fluorocarbons, and preferably allthree at the flow rates already set forth.

The flow rate for the ammonia will usually be at least about 2 sccm, andshould normally not exceed about 6 sccm. An upper limit flow rate ofabout 5 sccm is generally preferred. A flow rate range for the ammoniaof about 2 sccm to about 4 sccm is especially desirable. The flow ratesof both ammonia and the fluorocarbon(s) may be adjusted so as to yieldthe flow rate ratios previously described. An ammonia flow rate aboveabout 8 sccm is generally not preferred because at this rate theresultant reactant mixture can sometimes cause loss of selectivity tothe gate stack and/or spacer, and may also result in the etched openingnot being self-aligned to the gate stacks and/or the side wall spacers.

One or more of the fluorocarbons and the ammonia may be introduced intothe reaction chamber substantially simultaneously, or successively. Theorder of introduction should be consistent with the invention's goals ofeliminating etch stop, while providing a SAC opening 27 to the substrate12 in the device 10.

Other etchant gases which may be introduced into the reaction chambertogether with the foregoing ammonia and fluorocarbon(s) can includeoxygen, nitrogen and other compounds which are generally available inplasma etching.

After the etching process is complete such that the self-aligned contactopening 27 is formed, then the photoresist mask layer may be removedusing available methods.

As a result of the invention, the device shown in FIG. 3 has aself-aligned contact opening 27 that is formed without etch stopproblems. Moreover, the reactant mixture of fluorocarbon(s) and ammoniaproduces a thin protective layer 35 along the sides of the contactopening 27 defined by the sides of the insulative layer and the sidewall spacers 17. The protective layer 35 is a polymeric material formedas a result of the reaction between the reactant mixture and theinsulative layer and the side wall spacers, respectively. Formation ofthis protective layer 35 helps to prevent erosion and destruction of theside wall spacers during the etching process and thereafter, and istherefore desirable. The protective layer 35 is typically on the orderof just a few Angstroms in thickness, e.g. about 5- 50 Angstroms.

In contrast to the side wall spacers, substantially no layer is formedat the bottom of the opening 27. Without being bound by any particulartheory, it appears that any de minimis layer of residue that may beformed is rather quickly eliminated as a result of continuous contactwith the reactant mixture of the invention. Perhaps this is due at leastin part to the differing chemical components which make up theinsulative layer, in contrast to the side wall spacers upon theprotective layer 35 is formed.

A further embodiment of the invention is shown now with reference toFIG. 4. A conductive plug 37 may be formed in the contact opening 27after completion of the etching process. The conductive plug may beformed using a conductive metal such as tungsten, for example, usingtungsten hexafluoride (WF₆) and silane (SiH₄) using available depositiontechniques. Formation of the conductive plug may be proceeded bytitanium deposition and annealing to coat the inside of the contactopening 27 in which the plug is formed. Titanium deposition will form athin contact layer 39, e.g. about 5-50 Angstroms, over the active region19 of the substrate 12. This contact layer will in turn act as aprotective barrier to prevent free fluorine and tungsten atoms frompenetrating into the substrate 12 at the active region site 19 duringformation of the conductive plug 37.

After deposition of the conductive plug 37, the top of the plug may beco-planarized with the top of the insulative layer 21 using chemicalmechanical planarization (CMP) techniques, if desired. An optionalconductive metal runner can also be provided over the plug 37 usingavailable materials, e.g. aluminum, and methods (not shown in FIG. 4).As a result of the method and composition of the invention using thereactant mixture to etch the contact opening 27, the conductive plug 37adheres more effectively inside the contact opening 27. In particular,the protective layer 35 prevents erosion of the side wall spacers 17which could materially detract from the performance of the plug 37 andthe gate stacks 15.

The following examples illustrate certain preferred embodiments of theinvention, but should not be construed as limiting the scope thereof.

EXAMPLE 1

In this example, a self-aligned contact opening was formed in the deviceillustrated in FIG. 3. Plasma etching was conducted in a reactionchamber set at 600 watts, 45 milliTorr operating pressure, and 40 Gauss.Operating temperature was in the range of 0 to 50 degrees C. Thefollowing fluorocarbons were introduced into the reaction chambertogether with ammonia, at the following flow rates: CF₄ 18 sccm CHF₃ 40sccm CH₂F₂ 13 scccm NH₃  4 sccm

Under the foregoing conditions, a self-aligned contact opening wasformed in the device shown in FIG. 3 without etch stop.

EXAMPLE 2

In this example, the same operating parameters and reaction conditionswere utilized as set forth in Example 1, except that the flow rate ofammonia (NH₃) was 2 sccm. Under these conditions, a suitableself-aligned contact opening was also formed without etch stop.

EXAMPLE 3 (COMPARATIVE EXAMPLE)

In this example, the same operating parameters and reaction conditionswere utilized as set forth in Example 1, except that the flow rate ofammonia (NH₃) was 8 sccm. Under these conditions, loss of etchselectivity to gate stack and sidewall spacer was observed.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

In this example, the same operating parameters and reaction conditionswere utilized as set forth in Example 1, except that the flow rate ofammonia (NH₃) was 0 sccm. Under these conditions, etch stop wasobserved.

The foregoing description is illustrative of exemplary embodiments whichachieve the objects, features and advantages of the present invention.It should be apparent that many changes, modifications, substitutionsmay be made to the described embodiments without departing from thespirit or scope of the invention. The invention is not to be consideredas limited by the foregoing description or embodiments, but is onlylimited by the construed scope of the appended claims.

1. A method of forming a contact opening in an insulative layer formedover a substrate in a semiconductor device, said method comprising:etching said insulative layer with an etching composition consisting ofammonia and at least one fluorocarbon so as to form said contactopening, wherein the flow rate ratio of said at least one fluorocarbonto said ammonia is from about 2:1 to about 40:1, and said flow rate ofsaid ammonia is in the range from about 2 sccm to about 6 sccm.
 2. Themethod of claim 1, wherein said method is performed to produce aself-aligned contact opening, said opening is self-aligned between twoadjacent gate stack structures with side wall spacers.
 3. The method ofclaim 1, wherein said etching includes plasma etching.
 4. The method ofclaim 3, wherein said etching is performed within a temperature range ofabout −50 to about 80 degrees Celsius.
 5. The method of claim 4, whereinsaid etching is performed within a temperature range of about 0 to about50 degrees Celsius.
 6. The method of claim 4, wherein said etching isperformed at an operating pressure of about 25 to about 60 milliTorrs.7. The method of claim 4, wherein said etching is performed at anoperating pressure of about 40 to about 50 milliTorrs.
 8. The method ofclaim 1, wherein said etching is performed through a patternedphotoresist mask.
 9. The method of claim 1, wherein said fluorocarbon isat least one member selected from the group consisting of fluorinatedcarbons, fluorohydrocarbons, chlorofluorocarbons andchlorofluorohydrocarbons.
 10. The method of claim 9, wherein said atleast one fluorocarbon is at least one member selected from the groupconsisting of C₄F₈, C₄F₆, C₅F₈, CF₄, C₂F₆, C₃F₈, CHF₃, and CH₂F₂. 11.The method of claim 10, wherein said at least one fluorocarbon is atleast one member selected from the group consisting of CF₄, CHF₃, andCH₂F₂.
 12. The method of claim 1, wherein said method is performedwithout forming an etch stop.
 13. The method of claim 2, wherein saidside wall spacers remain unetched during formation of said self-alignedcontact opening.
 14. The method of claim 9, wherein said at least onefluorocarbon and said ammonia are flowed into a reaction chambercontaining said semiconductor device such that the flow rate ratio ofsaid at least one fluorocarbon to said ammonia is not less than about3:1.
 15. The method of claim 15, wherein the flow rate ratio of said atleast one fluorocarbon to said ammonia is within the range of about 3:1to about 20:1.
 16. The method of claim 16, wherein said flow rate ratiois within the range of about 4:1 to about 10:1.
 17. The method of claim11, wherein said at least one fluorocarbon is at least two membersselected from the group of CF₄, CHF3, and CH₂F₂.
 18. The method of claim17, wherein said at least one fluorocarbon comprises CF₄, CHF3, andCH₂F₂.
 19. The method of claim 11, wherein said at least onefluorocarbon is CF₄ which is flowed into a reaction chamber at a flowrate of about 15 to about 20 sccm.
 20. The method of claim 17, whereinsaid CF₄ is flowed into a reaction chamber at a flow rate of about 18sccm.
 21. The method of claim 11, wherein said at least one fluorocarbonis CHF₃ which is flowed into a reaction chamber at a flow rate of about35 to about 45 sccm.
 22. The method of claim 21, wherein said CHF₃ isflowed into a reaction chamber at a flow rate of about 40 sccm.
 23. Themethod of claim 11, wherein said at least one fluorocarbon is CH₂F₂which is flowed into a reaction chamber at a flow rate of about 10 toabout 15 sccm.
 24. The method of claim 23, wherein said CH₂F₂ isintroduced at a flow rate of about 13 sccm.